It has previously been proposed to make ballast elements for fluorescent lamps by joining together several T-shaped parts, arranged as a laminar stack, into a U-shaped core which surrounds the windings of the ballast. The legs and cross elements of the T-shaped core elements are surrounded by the legs of the U-shaped outer core element, and the dimensions are so arranged that two air gaps are formed which, to maintain their dimensions, may be filled with non-magnetic materials. The entire assembly is usually held together by a clamping bar.
Inductive units with high magnetic leakage are used frequently in applications in which the relationship between no-load and full-load voltage of transformers cannot be established by the internal voltage drop of a transformer. Leakage inductance units with a surrounding core are frequently constructed in form of a transformer in which magnetic shunts are located between with windings. Such a construction can be used, for example, if the element is to be suitable for two-line voltages, for example for either 110 V or 220 V. When connected for 220 V, the two windings are connected such that the magnetic flux from the respective windings will buck each other, so that the overall effect will be that of a choke; when used for 110 V, the two windings are serially connected so that, with the same construction, a high magnetic leakage transformer is obtained. One construction of this type is described in German Pat. No. 2,055,596. In this construction, a U-shaped laminar stack surrounding core element surrounds a laminar central core which is constructed of two T-shaped core portions. The cross bar of the T-shaped core portion is slightly smaller than the dimension between the longitudinal legs of the U-shaped outer core. The surrounding outer core cross elements match to and fit on the internal surfaces of the longitudinal legs of the U-shaped surrounding core. The smaller dimension of the cross bar then leaves an air gap in the shunt. This arrangement results in air gaps of predetermined size. The core laminae usually are punched. To permit proper punching under mass production conditions, and to accomodate tolerances in manufacture, the air gap must have a certain minimum size. To obtain a predetermined design impedance of this inductive element, it is necessary to match the number of turns of the windings thereon, that is, of the two windings which may be termed primary and secondary windings, to the size of the air gap. The actual size of the air gap, however, will depend not only on the design size but also on the accuracy of manufacture and of the punching tool itself. This causes difficulty in mass production since, if a certain size of air gap is determined to be used with a certain number of windings, the design of the winding must be arranged to accomodate worst-case conditions when punching the lamellae or transformer sheets on which the windings are to be placed.
The dependence of a design parameter of the actual size of the air gap leads to increased cost in manufacture of such a high leakage inductance transformer, since more material than is actually necessary has to be provided for; additionally, testing and re-arranging of the number of turns of the windings as actual air gaps change increases the manufacturing time. An air gap of predetermined size has the disadvantage that, as the punching tool becomes worn, the actual air gap due to inaccurate punching will change from the design size thereof so that, to maintain the predetermined design impedance of the inductance unit, the number of turns of the windings has to be changed, since there is no possibility to change the air gap once the lamellae have been punched. The construction has the additional disadvantage that the windings are placed above each other, since their common winding axis is at right angles to the assembly surface of the inductance unit, which causes heating of the upper winding by current flowing through the lower one. Thus, the design of the windings must take into consideration the mutual heat transfer between the windings due to current flow therethrough.